Need to test your car battery? We are a team of ASE certified mechanics that
have created this guide for you to help you test your own battery without the
aid of a tester. This test we also use in our shop which is easy any only takes about
What Goes Wrong?
Your car battery is designed as a chemical reactor which uses lead plates to
generate voltage. Over time (3 to 4 years) the batteries ability to create
voltage diminishes. A battery can fail in one of two ways, the first way is very quickly in
others word you go out to your car and no lights will come on and the car is
dead. In this case a jump start may not even help because the battery is so far gone it just dumps (pulls
in) all the voltage supplied to it.
The second way the battery can go bad is a slow decline of voltage which
results in slow than normal engine cranking, you will notice the starter sounding a little different.
Once you notice this it's up to you to be preventive and change the battery before total failure occurs.
The following test will help you discover a weak battery.
Let's Get Started!
When working with a battery wear protective eye wear and gloves. Stand clear
of the battery while the engine is being cranked over.
Step 1: Battery Test and Inspection
Open the you car hood to locate the battery and check its condition (some batteries can be located in the trunk
or under the rear seat). You are looking for leakage and bulging of the battery
sides which are an indication of warped internal plates that can short circuit.
You are also looking for corrosion at the positive and negative terminals which can eat away
at the metals in the battery cables and will cause a connection problem. To
start the battery test turn the headlights on and leave them on for about 15 minutes
(do not start the engine).
Step 2: Load Test the Battery
While the headlights are still on crank the engine over and observe the headlight bulb brightness.
The bulbs should only dim slightly when using the starter. If the headlights dim way down and go out or you notice the engine
is cranked over slowly followed by a machine gun style of clicking noise
must be replaced because it cannot supply the voltage needed to operate the
car. If the battery is good it will withstand this load test.
Car Battery Test Video
Here is a video of the car battery load test being performed.
A non "deep cycle" battery must have a rest period or "downtime" to allow the plates inside
each cell time to cool. A battery subjected to a complete discharge will
fail prematurely unless it is a deep cycle battery. All batteries are hazardous material
and must be disposed of at a local parts store or recycle center.
If the battery warning light has been on while driving the car
will need to test the alternator first. This is because the battery might
not be the problem if it is not receiving a proper charge from the alternator. If you have a
new battery and it goes dead overnight
something in the electrical system is draining the battery down.
Reasons a Battery Will Fail
Battery is more than three years old
The alternator is not charging or overcharging the electrical system
There is a electrical (parasitic) draw on the system